We have been studying alterations in expression of MMTV proviral and differentiation-specific genes such as ccasein and Alpha-lactalbumin at different points in mammary gland tumorigenesis in a "clean" inbred mouse strain (C3H/Sm). This strain is of interest because "normal" expression of MMTR RNA transcripts in mammary gland is not accompained by the appearance of viral structural proteins or by virions. Nevertheless mammary tumorigenesis, either experimentally induced or spontaneous, results in a significant increase in the abundance of these MMTV transcripts, especially a 2.2 kb RNA containing only MMTV long terminal repeat (LTR) sequences. This transformation-related increase in abundance of MMTV LTR RNA has been extended to include preneoplastic mammary lesions (hyperplastic alveolar nodules) in C3H/Sm mice as well. Outgrowth lines of these preneoplastic lesions possess enhanced activities of their casein and Alpha-lactalbumin genes in the absence of hormonal stimulation and release humoral factors which profoundly affect the development and differentiation of normal mammary tissue in mammary fat pads distal to their location in the host. We have tentatively identified several proteins encoded by MMTV LTR RNA which are present in these tumors and are abundantly represented in purified preparations of MMTV preprocapside (intracytoplasmic A particles). These proteins bind strongly to nucleic acids and have the ability to unwind native double-stranded DNA. Competition studies indicate that these LTR ORF proteins show definite specificity for binding to LTR DNA sequences. Studies are in progress to determine whether these proteins have trans-acting potential for gene expression.